I would like to save the wheelRevs every time the power goes off..
How will you detect that power is going to go away, prior to it going away and terminating the sketch?
wheelRevs is an unsigned long. It is easy enough to extract the 4 bytes that make up that long, with bit shifting and masking or using a union. Once you have the 4 bytes, store them in 4 addresses in the EEPROM.
On start up, read the 4 addresses, and reassemble the unsigned long, again with bit shifting and masking or a union.
One thing you could do is to measure the time between ISR calls. When the time exceeds some value, the vehicle is traveling very slowly/stopped. Save the data then. If the power is lost, you have saved data. If the vehicle starts moving again, you've wasted a write cycle, but that might be OK.
moreover I will be having additional 4 A, B, C, D Trip meters.. So I will have to save wheelRevs for them too...
You save these exactly the same way. Setting a trip meter to 0 simply means recording the number of wheel revs at that time. Each time you want to display that trip distance, subtract the starting wheelRevs value from the current wheelRevs value to get revs since reset, then multiply by circumference to get distance.
These are even easier to deal with as they only need to be saved when the set/reset button is pressed, not on power down.